This invention relates generally to an improved electrically biased transfer roll for an electrostatic reproduction machine. More particularly, the invention relates to a transfer roll having apparatus for heating the roll so as to improve the transfer of a developed image from a photoreceptor to a support member such as a sheet of paper during operation of the machine.
In the practice of xerography as described in U.S. Pat. No. 2,297,691 of Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate (photoreceptor) is electrostatically charged uniformly over its surface, and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern or electrostatic latent image in conformity with the configuration of the original pattern.
The latent electrostatic image is developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited, and where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the image of the original being produced. The powder image is subsequently transferred to a sheet of paper or other transfer member, and suitably affixed thereto to form a permanent copy.
The latest concept for electrostatic reproduction machines utilizes high speed flash exposure of the document, and a moving photoconductive material in the form of an endless belt which is continuously charged. Additionally, such reproduction machines are provided with a developing system which supplies toner particles in relatively large quantities for solid area coverage, such as a magnetic brush developing apparatus. Thus, after the belt passes the magnetic brush assembly, for example, a xerographic powder image is formed on the belt which corresponds to the electrostatic latent image. This powder image is then transferred to a support member (e.g., a sheet of paper) to which it is fused by a fusing assembly whereby the powder image is caused to adhere to the support surface permanently.
The latest electrostatic reproduction machines are high speed machines which print copies at a rate substantially in excess of any previous electrostatic reproduction machines, and are intended to compete with other types of printing machines, e.g., offset printing machines. Because of this, it is desired that the quality of the copies made, be extremely high. Important to high quality copies is an effective transfer apparatus for transferring the developed image from the photoreceptor to the support member. An electrically biased transfer roll includes a conductive core connected to an electrical source, and an elastomeric layer surrounding the core. The elastomeric layer contacts the paper during transfer, and an electric field is generated to transfer the developed electrostatic image from the photoreceptor to the paper. For optimum transfer, the resistivity of the transfer roll must be within a certain range. It has been found that ambient conditions such as temperature and humidity vary the resistivity of the transfer roll. In that embodiment of the transfer roll in which the transfer roll current is kept constant, it is desirable to control the resistivity to approximately within a factor of 8 or less. Many materials used for transfer rolls, however, have variations of resistivity larger than this factor in the temperature and humidity ranges in which the machine is expected to be operated. With many of these materials, temperature has been identified as the most significant variable contributing to this large variation in the resistivity of the transfer roll.